The present invention relates to a system and method for controlling an internal combustion engine and more particularly relates to a system and method for controlling the ignition timing of the engine which avoid the disadvantages due to possible malfunction of an ignition timing control system for a cylinder or a cylinder group.
A Japanese Patent Application Unexamined Open Publication No. Sho 53-56429 published on May 22, 1978 discloses a system for controlling the ignition timing of an internal combustion engine in which a plurality of pressure-responsive sensors (combustion pressure vibration sensors) are installed within corresponding engine cylinders and the ignition timing for each engine cylinder is individually corrected on the basis of the readings of the corresponding pressure-responsive sensor so that a crank angular position at which the pressure in the corresponding engine cylinder reaches a maximum coincides with a target value.
In more detail, the crank angular position .theta..sub.pmas at which the pressure in each engine cylinder reaches its maximum value usually falls at a crank angular position between 10.degree. and 20.degree. after top dead center (ATDC) (slightly later than the top dead center) although the crank angular position .theta..sub.pmas will vary slightly according to the model of the engine. The crank angular position .theta..sub.pmas can be adjusted as the ignition timing for the corresponding cylinder changes.
Therefore, the crank angular position .theta..sub.pmas is detected from the pressure in the corresponding cylinder and the ignition timing is controlled according to the measured crank angular position .theta..sub.pmas so that the crank angular position .theta..sub.pmas is adjusted to match a target angle value within a range from 10.degree. to 20.degree. ATDC, thereby maximizing the output torque of the engine and enhancing combustion efficiency.
However, in the ignition timing control system exemplified in the above-identified Japanese Patent Application, the relationship between each engine cylinder and the corresponding pressure-responsive sensor is definitely fixed. That is to say, the ignition timing control for the particular engine cylinder is carried out solely on the basis of the detected output of the single, non-adjustable pressure-responsive sensor.
Hence, if any of the pressure-responsive sensors should fail due to structural disruption, electrical disconnection, or short-circuiting, the ignition timing of the corresponding cylinder will accordingly be erroneous.
If the ignition timing is retarded beyond the normal ignition timing angle value, output torque is accordingly reduced, fuel consumption is increased, and an excessive increase in exhaust gas temperature results. On the other hand, if the ignition timing is advanced beyond the normal ignition timing angle value, engine knocking occurs. If the ignition timing is excessively advanced, the knocking may become so intense that, in the worst case, destruction of the engine would result.